A relevant protection device for vacuum-operated hoists is known from JP 06 270 086 A (Abstract). This comprises a pilot or servo valve that is controllable with the aid of a pressure differential and a weight detection unit when a load is lifted. This pilot valve prevents the vacuum from being able to be switched off inadvertently while a load is suspended.
Such vacuum-operated hoists have diverse use for the lifting of articles with large surface areas, e.g., metal sheets, glass plates, etc., but also of paper bags for bulk materials. They include in many embodiments a large number of diaphragm lifters or suction cups with lid seals that can be evacuated after placement on the article to be lifted.
Immediately after placement of said suction cups, they can already sit sealingly on the surface. During placement, air may be displaced from the space between the suction cup and the surface of the load. This generates a certain displacement vacuum. If the load is lifted already before evacuation or before the vacuum is turned on, it is possibly even still held in horizontal orientation. However, if the load is rotated to the vertical or even positioned at a slight angle, the load may be dropped if the displacement vacuum is no longer sufficient. This practical operational problem occurs primarily with loads whose weight is clearly the maximum liftable load limit. Weights near the nominal load of the hoist usually cannot even be lifted with the displacement vacuum alone. It is precisely this random occurrence of falling loads that makes the identification of the problem and the remedies more difficult.
EP 0 108 725 B1 describes another pneumatic lifting device with safety characteristics in which a mechanical load detection device (sensor) can control the application of a vacuum. However, with it is substantially guaranteed that the vacuum is not turned on before the application of the suction cups on a surface of a load.
DE 101 39 203 A1 discloses a device for handling loads with the aid of a suction cup lifting device. By means of a strain gauge measurement cell arrangement lying in the lift force flow, the weight of any suspended load is detected. The electrical signal resulting from the current weight is converted into a lifting or holding force sufficient for the static holding of the suspended load. Activation forces of a manual control lever are mechanically introduced into the measurements cell arrangement such that they positively or negatively overlay the weight dependent signal. A simulated raising of the suspended load (overlay of a positive signal) by raising the control lever in the lift direction results in further lifting of the load already picked up and held, whereas this can be lowered again by apparent reduction of the load using the control lever (overlay of a negative signal by pushing the lever down). The aforementioned document does not express itself with regard to monitoring the actual lifting process.